Heterodyne detection is a well-known method of optical signal processing. It is described, for example, by Vanderlugt in Optical Signal Processing (John Wiley & Sons, New York, 1992), Chapters 9-10, which are incorporated herein by reference. Typically, to perform heterodyne detection, a laser beam is split into a probe beam and a reference beam. The reference beam, of amplitude A0, is frequency-shifted by a known carrier frequency fc, typically using an acousto-optic modulator operating in the radio frequency (RF) range. The probe beam is incident on the sample, and is modified in amplitude and phase as a result, so that the beam reflected (or transmitted) by the sample has amplitude A1(t) and phase φ(t). The probe and reference beams are recombined and mutually interfere to give an optical signal whose intensity has the form:I(t)=A02+A12+2A0A1(t) cos [2πfct−φ(t)]  (1)
This combined signal is incident on a detector, and the detector output is filtered to extract the signal component at frequency fc. This heterodyne component is linear in the amplitude change A1(t) caused by the sample, and also contains the phase change data Φ(t). Therefore, information regarding the structure and characteristics of the sample is typically more easily extracted from the heterodyne signal than from simple (homodyne) intensity-based detection.
Interferometric measurements are known in the art of optical inspection of patterned substrates, such as semiconductor wafers. For example, U.S. Pat. No. 6,052,478, whose disclosure is incorporated herein by reference, describes an automated photomask inspection apparatus that uses transmitted or reflected interferometry to measure phase shifts produced by such masks. Variations in the phase shifts are indicative of defects due to undesired thickness variations in the photomask.